The present invention relates to sintered silicon carbide with graphite added thereto and to a sintered composite containing the same used for mechanical seals and the like. More particularly, the present invention relates to sintered silicon carbide with graphite added thereto and to a sintered composite containing the same which are endowed with excellent lubrication properties, and which provide excellent rubbing materials for such mechanical seals that are used under dry conditions, that are used under conditions in which the sliding surfaces become temporarily dry during operation, or that are used in combination with seal fluid having no lubrication ability, and which provide excellent material for bearings which produce temporary dry conditions.
Conventionally, cemented carbide, sintered alumina, sintered silicon carbide, and the like have been used as rubbing materials for mechanical seals. Among these materials, sintered silicon carbide has been most widely used, as it is endowed with good thermal conductivity, low coefficient of thermal expansion, high hardness, excellent wear resistance, excellent corrosion resistance, and excellent oxidation resistance. However, under dry conditions of gas or steam, sintered silicon carbide used as a rubbing material for stationary and rotating rings may cause seizure, scoring, generation of thermal cracks or abnormal wear, as sintered silicon carbide has poor lubrication ability. In addition, the same problems may be caused in temporary dry conditions, as experienced when hot water is used. Moreover, in fluids having poor lubrication ability such as water, rubbing materials may adhere to each other. This phenomenon, which is known as linking, may lead to damage to the mechanical seal itself.
Furthermore, sintered silicon carbide is a material difficult to work due to its high hardness, and causes problems such as generation of defectives during processing and high cost of processing.
In order to solve the above-mentioned problems characteristic to sintered silicon carbides, there have been suggested materials having improved friction and wear properties. For example, Japanese Patent Application Laid-Open (kokai) No. 61-132575 (the disclosure of which is herein incorporated by reference) discloses a silicon carbide composite which uses a sintered silicon carbide, which is suitable for producing a composite together with synthetic resins such as fluororesins. However, as described in examples of the publication, this silicon carbide composite has extremely poor bending strength compared with that of sintered silicon carbide prepared by way of sintering under atmospheric pressure, and has poor heat and corrosion resistance as it is a composite containing resin, making it unsuitable as a rubbing material for a mechanical seal used to seal corrosive or high temperature fluid.
Japanese Patent Application Laid-Open (kokai) No. 60-141676 (the disclosure of which is herein incorporated by reference) discloses a silicon carbide/graphite/carbon composite ceramic material formed of approximately 1-48 wt. % of graphite carbon having an average grain size of not more than 8 .mu.m, about 0.5-5.0 wt. % of amorphous carbon, and the remainder silicon carbide. Japanese Patent Application Laid-Open (kokai) No. 63-260861 (the disclosure of which is herein incorporated by reference) discloses a silicon carbide/graphite ceramic composite having self-lubrication ability; i.e., a silicon carbide composite with graphite added thereto consisting of granules of graphite grains having primary grains of 5-8 .mu.m, as well as a process of producing the same. However, none of these materials have sufficient lubrication ability or bending strength.
The object of the present invention is to provide sintered silicon carbide with graphite added thereto (hereinafter referred to as "graphite-added sintered silicon carbide) and a sintered composite containing the same which can solve the above-mentioned problems characteristic to conventional silicon carbide composites, and which exhibit excellent rubbing characteristics, particularly under dry conditions and under non-lubricant conditions, as well as mechanical seals prepared therefrom.